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Abstract:

An image forming apparatus detects a plurality of times by changing
velocity of a photosensitive drum, color misregistration generated when a
developing roller separates from a photosensitive drum and calculates a
relation between the velocity of the photosensitive drum and the color
misregistration. A velocity of the photosensitive drum is changed and an
arbitrary peripheral velocity difference is set between the
photosensitive drum and the intermediate transfer belt based on the
calculated result.

Claims:

1. An image forming apparatus including an image forming unit comprising a
plurality of image bearing members, a plurality of developing units
capable of coming into contact with and separating from each of the
plurality of image bearing members, an intermediate transfer member onto
which toner images developed on the plurality of image bearing members by
the plurality of developing units are transferred, and a transfer member
which forms a nip portion with the image bearing member by sandwiching
the intermediate transfer member, the image forming apparatus
comprising:a pattern forming unit configured to form on the intermediate
transfer member by employing the image forming unit a pattern for
detecting positional deviation including a first color mark formed in a
stable state in which toner enters all of the nip portions of the
plurality of image bearing members, and a second color mark formed in a
fluctuating state in which toner enters apart of the nip portions of the
plurality of image bearing members;a detection unit configured to detect
positions of the first color mark and the second color mark included in
the pattern for detecting positional deviation; anda correction unit
configured to correct a relative velocity between the image bearing
member and the intermediate transfer member based on a detection result
of the detection unit,wherein the pattern forming unit causes the image
forming unit to form a first pattern and a second pattern as the patterns
at a plurality of the relative velocities, andwherein the correction unit
corrects the relative velocity based on a position of the first color
mark and a position of the second color mark included in the first
pattern and a position of the first color mark and a position of the
second color mark included in the second pattern, detected by the
detection unit.

2. An image forming apparatus including an image forming unit comprising a
plurality of image bearing members, a plurality of developing units
capable of coming into contact with and separating from each of the
plurality of image bearing members, an intermediate transfer member onto
which toner images developed on the plurality of image bearing members by
the plurality of developing units are transferred, and a transfer member
which forms a nip portion with the image bearing member by sandwiching
the intermediate transfer member, the image forming apparatus
comprising:a pattern forming unit configured to form on the intermediate
transfer member by employing the image forming unit a pattern for
detecting positional deviation including a first color mark formed in a
stable state in which all of the plurality of developing units are in
contact with the plurality of image bearing members, and a second color
mark formed in a fluctuating state in which a part of the plurality of
developing units is separated from or is in contact with the plurality of
image bearing members;a detection unit configured to detect positions of
the first color mark and the second color mark included in the pattern
for detecting positional deviation; anda correction unit configured to
correct a relative velocity between the image bearing member and the
intermediate transfer member based on a detection result of the detection
unit,wherein the pattern forming unit causes the image forming unit to
form a first pattern and a second pattern as the patterns at a plurality
of the relative velocities, andwherein the correction unit corrects the
relative velocity between the image bearing member and the intermediate
transfer member based on a position of the first color mark and a
position of the second color mark included in the first pattern, and a
position of the first color mark and a position of the second color mark
included in the second pattern, detected by the detection unit.

3. The image forming apparatus according to claim 1, wherein the detection
unit detects a first positional deviation amount based on a position of
the first color mark and a position of the second color mark included in
the first pattern, and a second positional deviation amount based on a
position of the first color mark and a position of the second color mark
included in the second pattern, andwherein the correction unit corrects
the relative velocity between the image bearing member and the
intermediate transfer member based on the first positional deviation
amount and the second positional deviation amount.

4. The image forming apparatus according to claim 1, wherein the pattern
forming unit causes the image forming unit to form the first color mark
in a state in which the toner enters all of the nip portions by the
movement that the developing unit comes in contact with all of the
plurality of the image bearing members, and the pattern forming unit
causes the image forming unit to form the second color mark in a state in
which the toner enters one of the nip portions by the movement that the
developing unit of one of the plurality of image bearing members is
separated or comes in contact.

5. The image forming apparatus according to claim 1, wherein the pattern
forming unit causes the image forming unit to form a third pattern
corresponding to at least one of the first pattern and the second
pattern, which includes the first color mark and the second color mark,
in the stable state and at the relative velocity which is the same as
that of at least one of the patterns,wherein the correction unit corrects
the relative velocity based on a detection result of the first pattern,
the second pattern, and the third pattern.

6. The image forming apparatus according to claim 5, wherein the
correction unit extracts positional deviation amount of the first pattern
and second pattern, which is caused by velocity fluctuation of an
intermediate transfer and from which a direct current positional
deviation by a detection result of the third pattern is excluded, and the
correction unit corrects the relative velocity based on the extraction
result.

7. The image forming apparatus according to claim 1, wherein the pattern
forming unit forms the first pattern and the second pattern on a same
region in the intermediate transfer member in the image forming unit.

8. The image forming apparatus according to claim 1, wherein the
correction unit corrects a velocity of either the image bearing member or
the intermediate transfer member.

Description:

BACKGROUND OF THE INVENTION

[0001]1. Field of the Invention

[0002]The present invention relates to drive control of an image forming
apparatus which forms an image on a recording medium.

[0003]2. Description of the Related Art

[0004]Color misregistration is one of the criteria for determining output
image quality of a color image forming apparatus in which high-quality
image output is demanded. To reduce such color misregistration, the image
forming apparatus may form toner patches of each color on an intermediate
transfer belt and detect color misregistration using a registration
detection sensor to detect the position of the toner patches. The color
image forming apparatus then changes timing of forming each color image
on a photosensitive drum based on the detection result.

[0005]Further, velocity fluctuation of the intermediate transfer belt
causes color misregistration in an image forming apparatus which
sequentially activates image forming units including the photosensitive
drum. If velocity fluctuation is generated in a transfer conveyance belt
or the intermediate transfer belt, power applied on the belt from an
image bearing member becomes different at respective transfer nips of the
image forming units for each color. As a result, a pulling force or a
pressing force is applied on the belt between the transfer nips of the
image forming units of each color, which causes a difference in the
velocities of the belt passing through each of the transfer nips. Color
misregistration is thus generated. When peripheral velocities of the
photosensitive drum and the intermediate transfer belt are different, a
friction coefficient between the photosensitive drum and the intermediate
drum changes according to the presence or absence of toner in a primary
transfer nip portion. Such change in the friction coefficient also causes
a change in a tangential force, thus leading to generation of color
misregistration.

[0006]To solve such a problem, there is a technique for preventing
velocity fluctuation of the intermediate transfer belt from affecting the
image. More specifically, load fluctuation is generated when charging,
developing, and transferring processes are switched on and off in the
image forming unit. In such a technique, the processes are switched on
and off when a visualized image is not being transferred from the
photosensitive drum to the intermediate transfer member.

[0007]However, in the above-described method, time for performing the
charging and developing processes becomes longer, so that the lifetime of
the image forming unit becomes immoderately shortened.

[0008]Further, it has been determined by inventors that a relation between
the peripheral velocity difference of the photosensitive drum and the
intermediate transfer belt, and the color misregistration caused by the
velocity fluctuation of the intermediate transfer belt changes due to
other factors. An example of such factors is usage of the photosensitive
drum and the intermediate transfer belt. It is thus necessary to consider
the factors which affect the degree of change in the tangential force to
reduce color misregistration.

SUMMARY OF THE INVENTION

[0009]The present invention is directed to reducing color registration
without immoderately shortening the life of the image forming unit and by
flexibly suppressing velocity fluctuation of the intermediate transfer
belt generated while forming an image.

[0010]According to an aspect of the present invention, an image forming
apparatus includes an image forming unit comprising a plurality of image
bearing members, a plurality of developing units capable of coming into
contact with and separating from each of the plurality of image bearing
members, an intermediate transfer member onto which toner images
developed on the plurality of image bearing members by the plurality of
developing units are transferred, and a transfer member which forms a nip
portion with the image bearing member by sandwiching the intermediate
transfer member. The image forming apparatus further includes a pattern
forming unit configured to form on the intermediate transfer member by
employing the image forming unit a pattern for detecting misregistration
including a first color mark formed in a stable state in which toner
enters all of the nip portions of the plurality of image bearing members,
and a second color mark formed in a fluctuating state in which toner
enters a part of the nip portions of the plurality of image bearing
members, a detection unit configured to detect positions of the first
color mark and the second color mark included in the pattern for
detecting misregistration, and a correction unit configured to correct a
relative velocity between the image bearing member and the intermediate
transfer member based on a detection result of the detection unit. The
pattern forming unit forms a first pattern and a second pattern as the
patterns with respect to a plurality of the relative velocities, and the
correction unit corrects the relative velocity based on a position of the
first color mark and a position of the second color mark included in the
first pattern and a position of the first color mark and a position of
the second color mark included in the second pattern, detected by the
detection unit.

[0011]According to the present invention, color registration can be
reduced without immoderately shortening the life of the image forming
unit and by flexibly suppressing velocity fluctuation of the intermediate
transfer member generated while forming an image.

[0012]Further features and aspects of the present invention will become
apparent from the following detailed description of exemplary embodiments
with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]The accompanying drawings, which are incorporated in and constitute
a part of the specification, illustrate exemplary embodiments, features,
and aspects of the invention and, together with the description, serve to
explain the principles of the invention.

[0014]FIG. 1 is a cross-sectional view of a full-color image forming
apparatus according to an exemplary embodiment of the present invention.

[0015]FIG. 2 is a block diagram illustrating a configuration of an image
forming apparatus according to an exemplary embodiment of the present
invention.

[0016]FIGS. 3A, 3B, and 3C illustrate examples of a perspective view of an
intermediate transfer belt and a color misregistration detection
patterns.

[0017]FIG. 4 illustrates an example of fluctuation of torque on a drive
roller shaft which drives an intermediate transfer belt while printing,
with respect to time.

[0019]FIG. 6 illustrates an example of a relation between a peripheral
velocity difference between a photosensitive drum and an intermediate
transfer belt, and tangential force acting on a primary transfer nip.

[0020]FIGS. 7A, 7B, and 7C illustrate examples of generation of color
misregistration of a yellow toner image with respect to a black color
image when three letter size sheets are continuously printed.

[0021]FIG. 8 illustrates an example of a relation between the
photosensitive drum velocity and a color misregistration amount.

[0022]FIG. 9 is a flowchart illustrating a process for correcting the
photosensitive drum velocity.

[0024]FIG. 11 illustrates an example of a relation between the peripheral
velocity difference between a photosensitive drum and an intermediate
transfer belt, and color misregistration generated when there is velocity
fluctuation in the intermediate transfer belt.

DESCRIPTION OF THE EMBODIMENTS

[0025]The individual embodiments described below will be helpful in
understanding a variety of concepts of the present invention from the
generic to the more specific. Further, the technical scope of the present
invention is defined by the claims, and is not limited by the following
individual embodiments.

[0026]Various exemplary embodiments, features, and aspects of the
invention will be described in detail below with reference to the
drawings.

[0027]FIG. 1 is a schematic diagram illustrating a configuration of a
four-drum full-color image forming apparatus employing an intermediate
transfer belt, among image forming apparatuses according to an exemplary
embodiment of the present invention.

[0028]Referring to FIG. 1, a four-drum full-color image forming apparatus
1 includes a four-drum full-color image forming apparatus main body 2
(hereinafter referred to as an apparatus main body 2). The apparatus main
body 2 includes process cartridges PY, PM, PC, and Pbk for each of four
respective colors, i.e., yellow, magenta, cyan, and black. The apparatus
main body 2 further includes an intermediate transfer belt unit 31
comprising an intermediate transfer belt 30, and a fixing unit 25.

[0029]Each of the process cartridges is located on an outer
circumferential surface of respective photosensitive drums 26Y, 26M, 26C,
and 26Bk (i.e., on the image bearing member). Each process cartridge
includes a primary charging unit 50 which uniformly charges a surface of
each of the photosensitive drum 26. Further, the process cartridge
includes a developing unit 51 which develops an electrostatic latent
image on the surface of the photosensitive drum 26 formed by laser
exposure from laser exposure units 28Y, 28M, 28C, and 28Bk. The process
cartridges are arranged in parallel along the intermediate transfer belt
30.

[0030]A developing roller 54 inside the developing unit 51 causes the
entire developing unit 51 to separate from the photosensitive drum 26 and
thus stop rotating to prevent deterioration of developer. Further, a
primary transfer roller 52 is disposed opposite to the photosensitive
drum 26 to sandwich the intermediate transfer belt 30 with the
photosensitive drum 26. The primary transfer roller 52 forms a primary
transfer portion with the photosensitive drum 26. Furthermore, the
photosensitive drums 26Y, 26M, 26C, and 26Bk are driven by a drum driving
motor (not illustrated). The drum driving motor can be individually
installed for each photosensitive drum or can be shared by a plurality of
photosensitive drums. Moreover, the present exemplary embodiment can also
be applied to a photosensitive belt instead of the photosensitive drum as
described above.

[0031]The intermediate transfer belt unit 31 includes the intermediate
transfer belt 30, and a drive roller 100, a tension roller, and a
secondary transfer counter roller 108 around which the intermediate
transfer belt 30 is stretched. A belt drive motor 14 (not illustrated)
rotationally drives the drive roller 100, and the intermediate transfer
belt 30 is thus rotationally conveyed. The tension roller 105 can move in
a horizontal direction shown in FIG. 1 according to a length of the
intermediate transfer belt 30.

[0032]Further, there are two registration detection sensors 90 near the
tension roller 105 at both ends in a longitudinal direction of the
tension roller 105. The registration detection sensors 90 which detect
the toner patches on the intermediate transfer belt 30 are disposed
opposite to the image bearing member, and each detection sensor includes
a light emitting unit and a light receiving unit. The light emitting unit
of the registration detection sensor 90 irradiates with light the toner
image formed on the image bearing member or the image bearing member
itself, and the light receiving unit receives the reflected light. For
example, the registration detection sensor 90 irradiates a color
misregistration detection pattern (to be described below) with light and
receives the reflected light. In such a case, the registration detection
sensor 90 detects the position of the color misregistration detection
pattern or mark by a change in reflection of the image bearing member and
the color misregistration detection pattern.

[0034]The belt drive motor 14 is a drive unit for rotationally driving the
intermediate transfer belt 30 at a predetermined velocity by instruction
form an image forming control unit. Further, the drum drive motor is a
drive unit for rotationally driving all photosensitive drums 26 at a
predetermined velocity by instruction from the image forming control
unit.

[0035]FIG. 2 is a block diagram illustrating a control configuration of
the image forming apparatus according to an exemplary embodiment of the
present invention.

[0036]Referring to FIG. 2, the apparatus main body 2 illustrated in FIG. 1
receives an image signal (RGB signal or page description language data)
from an external host apparatus 10 such as a personal computer which is
communicably connected thereto. The apparatus main body 2 may also
receive the image signal from a document reading unit (not illustrated)
separately included therein. An image processing control unit 11 converts
the received image signal to a CMYK signal, performs gradation and
concentration correction on the converted CMYK signal, and generates an
exposure signal to be used by the laser exposure unit 28.

[0037]An image forming control unit 12 comprehensively controls the image
forming operation described below. The image forming control unit 12 also
controls the apparatus main body 2 when correcting the image forming
operation by using the registration detection sensors 90 and a mark
sensor 91. The image forming control unit 12 includes a central
processing unit (CPU) 121, a read-only memory (ROM) 122 which stores
programs to be executed by the CPU 121, and a random access memory (RAM)
123 which store various data when the CPU 121 performs control.

[0038]An image forming unit 13 includes the photosensitive drum 26
illustrated in FIG. 1, and a charging unit, a developing unit, a cleaning
unit, and an exposure unit which act on the photosensitive drum 26. One
image forming unit 13 or a plurality of the image forming units 13 is
disposed in the rotational direction of the intermediate transfer belt
30.

[0039]The belt drive motor 14 is the drive unit which adjusts a conveying
velocity of the intermediate transfer belt 30 in response to an
instruction from the image forming control unit 12. A registration
detection sensor unit 15 uses the registration detection sensors 90 to
detect the toner patches on the intermediate transfer belt 30. A mark
sensor detection unit 16 uses the mark sensor 91 to detect a position
indication mark disposed on the intermediate transfer belt 30.

[0040]The image forming operation performed by the above-described
four-drum full-color image forming apparatus 1 will be described below
with reference to FIG. 1. Upon start of the image forming operation, the
sheet feeding roller 21 feeds transfer materials P in the cassette 20.
The retard roller pair 22 separates the transfer materials P into each
sheet which is then conveyed to the registration roller pair 24 via the
conveyance roller pairs 23a and 23b. In parallel with the conveyance
operation of the transfer materials P, the surface of the photosensitive
drum 26Y in the yellow process cartridge PY is uniformly charged to a
negative polarity by the primary charging unit 50. The laser exposure
unit 28Y then exposes the photosensitive drum with image light, so that
the electrostatic latent image corresponding to a yellow image component
of the image signal is formed on the surface of the photosensitive drum
26Y.

[0041]The developing roller 54Y in the developing roller 51 is then
rotationally driven to come into contact with the photosensitive drum
26Y. The developing unit 51 develops the electrostatic latent image using
the negatively-charged yellow toner, and the electrostatic latent image
is thus visualized as a yellow toner image. The developing unit 51 can
also come into contact with the photosensitive drum 26 directly before
forming the electrostatic latent image. The primary transfer roller 52 on
which a primary transfer bias is applied then primarily transfers the
acquired yellow toner image onto the intermediate transfer belt 30. A
cleaner 53 removes residual toner adhering to the surface of the
photosensitive drum 26Y after the toner image is transferred.

[0042]Such series of toner image forming operation is also sequentially
performed in the other process cartridges PM, PC, and PBk. More
specifically, the color toner images formed on each of the respective
photosensitive drums 26 are primarily transferred at the primary transfer
portion and sequentially superimposed on the intermediate transfer belt
30. Upon completing the developing process, the developing roller 54
separates from the photosensitive drum 26 and stops rotating even when
the process cartridge located downstream of the conveyance path is
performing primary transfer. This is to prevent deterioration of the
developer. The contacting-separating sequence of the developing unit 51
will be described below with reference to FIG. 10.

[0043]The four-color toner image superimposed and transferred on the
intermediate transfer belt 30 is then shifted to the secondary transfer
portion by rotation of the intermediate transfer belt 30 in a direction
indicated by an arrow illustrated in FIG. 1. The transfer material P is
also conveyed to the secondary transfer portion in time with the image
transferred on the intermediate transfer belt 30. The secondary transfer
roller 27 then comes into contact with the intermediate transfer belt 30
by sandwiching the transfer material P, and secondarily transfers the
four-color toner image on the intermediate transfer belt 30 to the
transfer material P. The transfer material P on which the toner image is
thus transferred is conveyed to the fixing unit 25 where the toner image
is heated and press-fixed on the transfer material P. The discharge
roller pairs 61, 62, and 63 then discharge and mount the transfer
material P onto the apparatus main body 2.

[0046]FIG. 3A illustrates a perspective view of a configuration of the
intermediate transfer belt unit 31. Referring to FIG. 3A, the
intermediate transfer belt 30 is rotating in a direction illustrated by
an arrow at a speed V mm/s. A regulating rib 301 is adhered to both side
edges of an inner circumferential surface of the intermediate transfer
belt 30 according to the present exemplary embodiment. The regulating rib
301 which is regulated by a regulating flange (not illustrated) disposed
at both ends of the tension roller 105 prevents the intermediate transfer
belt 30 from meandering. Further, a transparent belt reinforcing tape 302
is adhered to both side edges of the outer circumferential surface of the
intermediate transfer belt 30 to prevent the intermediate transfer belt
30 from being damaged. The registration detection sensor 90 is a
reflective optical sensor for detecting an unfixed toner patch formed on
the intermediate transfer belt 30. According to the present exemplary
embodiment, the registration detection sensor 90 is disposed on each end
of the tension roller 105 in the longitudinal direction.

[0047]Color misregistration mechanism will be described below. A drive
transmission system which drives the intermediate transfer belt 30
includes a series of gears. Distortion of a gear tooth surface or a sheet
metal supporting the drive transmission system, or tipping of a shaft
supporting the gear due to a load torque causes a delay in drive
transmission. As a result, if the torque on a drive roller shaft driving
the intermediate transfer belt 30 fluctuates when the developing roller
54 comes into contact with or separates from the photosensitive drum 26,
velocity fluctuation is generated in the intermediate transfer belt 30.
The velocity fluctuation is generated when there is load torque
fluctuation and a change in a distortion amount of the drive transmission
system. The velocity fluctuation is not generated when the distortion
amount of the drive transmission system is constant owing to a regular
load torque.

[0048]If the peripheral velocity of the photosensitive drum 26 is less
than the peripheral velocity of the intermediate transfer belt 30, the
peripheral velocity of the intermediate transfer belt 30 increases when
the developing roller 54 is in contact with the photosensitive drum 26.
The peripheral velocity of the intermediate transfer belt 30 remains
constant when there is no torque fluctuation and decreases when the
developing roller 54 separates from the photosensitive drum 26.

[0049]On the contrary, if the peripheral velocity of the photosensitive
drum 26 is greater than that of the intermediate transfer belt 30, the
peripheral velocity of the intermediate transfer belt 30 decreases when
the developing roller 54 comes into contact with the photosensitive drum
26. The peripheral velocity of the intermediate transfer belt 30
increases when the developing roller 54 separates from the photosensitive
drum 26.

[0050]Causes of the velocity fluctuation of the intermediate transfer belt
30 will be described in detail below.

[0051](i) Velocity Fluctuation Due to Entry of Toner

FIG. 4 illustrates the load torque on the drive roller shaft when making a
print in a case where the peripheral velocity difference between the
photosensitive drum 26 and the intermediate transfer belt 30 is zero or
proximately zero. Further, FIG. 4 illustrates the load torque in a case
where the velocity of the photosensitive drum 26 is changed so that the
peripheral velocity difference is purposely generated. The "peripheral
velocity difference" indicates a difference between the velocity of the
photosensitive drum in a tangential line direction and the velocity of
the intermediate transfer belt at the primary transfer nip portion.

[0052]Referring to FIG. 4, line A indicates the load torque on the drive
roller shaft when the peripheral velocity of the photosensitive drum is
0.4% less than the peripheral velocity of the intermediate transfer belt.
Line B indicates the load torque when the peripheral velocity of the
photosensitive drum and the peripheral velocity of the intermediate
transfer belt are the same or proximately the same. Line C indicates the
load torque when the peripheral velocity of the photosensitive drum is
0.4% greater than the peripheral velocity of the intermediate transfer
belt. The "peripheral velocity of the photosensitive drum" is the
velocity of the photosensitive drum surface at the nip portion in the
tangential line direction. The "peripheral velocity of the intermediate
transfer belt" is the velocity of the intermediate transfer belt at the
nip portion in a conveying direction.

[0053]According to FIG. 4, transient torque fluctuation is generated when
there is a peripheral velocity difference between the photosensitive drum
26 and the intermediate transfer belt 30 while forming an image. The
torque fluctuation begins when the developing roller 54 inside the
developing unit 51 which is rotationally driven comes into contact with
the yellow photosensitive drum 26Y. The developing roller 54 of each
color located downstream of the developing roller 54Y then sequentially
comes into contact with the respective photosensitive drum 26Y. The
torque fluctuation ends after the black developing roller 54Bk comes into
contact with the photosensitive drum 26Bk. The torque fluctuation begins
again when the primary transfer of the yellow toner image ends and the
developing roller 54Y is separated from the photosensitive drum 26Y.

[0054]The torque fluctuation generated when the developing roller 54 comes
into contact with and is separated from the photosensitive drum 26 is
caused by the toner entering the primary transfer nip. The toner of the
developing roller 54Y adheres to the photosensitive drum 26Y as fogging
toner when the latent image is being formed. The fogging toner then
reaches the primary transfer nip portion between the photosensitive drum
26Y and the intermediate transfer belt 30.

[0055]FIG. 5 illustrates an example of a case where the tangential force
acts on the primary transfer nip. The "tangential force" is a force which
acts in the direction of the tangential line of the photosensitive drum
at the primary transfer nip portion. Referring to FIG. 5, a vertical drag
N acts on the primary transfer nip. The vertical drag N is expressed as a
sum of a primary transfer pressure Np which is a mechanical pressing
force, and an electrostatic attraction force Ne which is an electric
attraction force. Further, a tangential force F acting on the primary
transfer nip when there is a peripheral velocity difference is expressed
by equation (1). μ indicates a friction coefficient between the
photosensitive drum 26 and the intermediate transfer belt 30.

F=μ×(Np+Ne) (1)

If there are four photosensitive drums 26 for the four colors, the
tangential force F is generated in each primary transfer nip, and a
resultant force T of the tangential forces for each color acts on the
intermediate transfer belt 30.

[0056]Further, if the friction coefficients between the photosensitive
drum 26 and the intermediate transfer belt 30 is μ1 when there is no
toner in the primary transfer nip and μ2 when there is toner in the
primary nip, the relation between μ1 and μ2 is μ1>μ2.

[0057]When there is no toner in the primary transfer nip, a resultant
force T acting on the intermediate transfer belt 30 is expressed by
equation (2). According to equation (2), the load on the intermediate
transfer belt 30 is four times the load on the photosensitive drum 26.

T=μ1×(Np+Ne)×4 (2)

Upon start of the image forming operation, the developing roller 54Y comes
into contact with the yellow photosensitive drum 26Y, and the yellow
toner enters the yellow primary transfer nip. In such a case, a power T1
acting on the intermediate transfer belt 30 is expressed as equation (3).

T1=μ1×(Np+Ne)×3+μ2×(Np+Ne) (3)

The developing roller 54 of each color then sequentially comes into
contact with the respective photosensitive drum 26.

[0058]When the toner enters the primary transfer nip, the force acting on
the intermediate transfer belt 30 changes as expressed in an order of
equation (4), equation (5), and equation (6).

T2=μ1×(Np+Ne)×2+μ2×(Np+Ne)×2 (4)

T3=μ1×(Np+Ne)+μ2×(Np+Ne)×3 (5)

T4=μ2×(Np+Ne)×4 (6)

Since the relation between μ1 and μ2 is μ1>μ2, a relation
between the forces acting on the intermediate transfer belt 30 becomes
T1>T2>T3>T4.

[0059]If the peripheral velocity of the photosensitive drum 26 is less
than that of the intermediate transfer belt 30, the photosensitive drum
26 acts as a brake with respect to the intermediate transfer belt 30. In
such a case, as illustrated in FIG. 4, the torque on the drive roller
shaft increases at the start of the image forming operation when the
primary transfer roller 52 comes into contact with the photosensitive
drum 26 and applies the primary transfer bias.

[0060]A force T then acts on the intermediate transfer belt 30. The
developing roller 54 of each color then comes into contact with the
respective photosensitive drum 26, and the force acting on the
intermediate transfer belt 30 changes from T1 to T2 and to T3. The torque
on the drive roller shaft thus gradually decreases. The tangential force
stops fluctuating further when the toner enters the primary transfer nip
where the black toner image is primarily transferred and the force acting
on the intermediate transfer belt 30 becomes T4. As a result, the torque
on the drive roller shaft stops fluctuating.

[0061]When the primary transfer of the yellow toner image is completed and
the developing roller 54Y separates from the photosensitive drum 26Y,
there is no toner left in primary transfer nip where the yellow toner
image is primarily transferred. The force acting on the intermediate
transfer belt 30 thus becomes T3. The developing roller 54 of each color
then separates from the respective photosensitive drum 26, and the force
acting on the intermediate transfer belt 30 changes to T2, T1, and to T
and becomes greater. The torque on the drive roller shaft thus increases.

[0062]On the contrary, if the peripheral velocity of the photosensitive
drum (Vd) is greater than that of the intermediate transfer belt (Vb),
the photosensitive drum 26 assists the rotation of the intermediate
transfer belt 30. When the developing roller 54 of each color
sequentially comes into contact with the respective photosensitive drum
26, a force with which the photosensitive drum 26 assists the rotation of
the intermediate transfer belt 30 decreases. The torque of the drive
roller shaft thus gradually increases. After the primary transfer ends
and the developing roller 54 starts separating from the photosensitive
drum 26, the force with which the photosensitive drum 26 assists the
rotation of the intermediate transfer belt 30 increases. The torque on
the drive roller shaft thus decreases.

[0063](ii) Relation Between Velocity Fluctuation and a Size of the
Peripheral Velocity Difference

FIG. 6 illustrates a relation between the peripheral velocity difference
between the photosensitive drum 26 and the intermediate transfer belt 30
and the tangential force acting on the primary transfer nip. If the
peripheral velocity difference is small, the tangential force increases
along with the peripheral velocity difference. However, since the
friction coefficient μ changes according to the size of the peripheral
velocity difference, the tangential force becomes constant when the
peripheral velocity difference becomes greater.

[0064]If the peripheral velocity difference is zero or proximately zero,
the photosensitive drum 26 and the intermediate transfer belt 30 are in
rolling contact, so that the friction coefficient is zero. However, if
the peripheral velocity difference is small, the photosensitive drum 26
and the intermediate transfer belt 30 are in both rolling contact and
sliding contact. The friction coefficient thus increases as the
peripheral velocity difference increases. When the peripheral velocity
difference becomes greater than a predetermined value, the photosensitive
drum 26 and the intermediate transfer belt 30 come into sliding contact,
and the friction coefficient becomes constant. As a result, the relation
between the peripheral velocity difference and the tangential force
becomes as illustrated in FIG. 6.

[0065](iii) Velocity Fluctuation and the Degree of Usage

The friction coefficient μ increases as surface roughness of the
intermediate transfer belt 30 increases. Flaws generated on the
intermediate transfer belt 30 due to usage causes the increase in the
surface roughness. As a result, as illustrated in FIG. 6, the tangential
force F is greater in a used intermediate transfer belt as compared to a
new intermediate transfer belt even when the peripheral velocity
differences are the same in both cases. Further, the same can be said for
the photosensitive drum 26. The usage status indicates the degree of
usage, and an increase in the degree of usage indicates deterioration of
the intermediate transfer belt caused by heavy usage.

[0066](iv) Velocity Fluctuation Due to Other Factors

Other examples of factors which cause velocity fluctuation of the
intermediate transfer belt 30 are the environment of the image forming
apparatus (e.g., temperature and humidity), and outside diameter
tolerance (manufacturing error) of the drive roller 100 which is
attributable to manufacturing conditions. Further, aged deterioration of
the image forming apparatus may cause velocity fluctuation of the
intermediate transfer belt 30. The degree of velocity fluctuation due to
factors described in (i), (ii) and (iii) changes according to such
factors. To address the fluctuation, the image forming apparatus
according to the present exemplary embodiment flexibly responds to the
various factors and reduces velocity fluctuation of the intermediate
transfer member generated during the image forming operation. Color
misregistration is thus reduced.

[0067]A relation between the velocity fluctuation of the intermediate
transfer belt 30 and color misregistration will be described below. FIGS.
7A, 7B, and 7C illustrate misregistration of the yellow toner image with
respect to the black toner image when three letter-size sheets are
continuously output. In the example, the peripheral velocity of the
photosensitive drum is less than the peripheral velocity of the
intermediate transfer belt. FIG. 7A illustrates color misregistration in
the first sheet, FIG. 7B in the second sheet, and FIG. 7C in the third
sheet.

[0068]Referring to FIGS. 7A, 7B, and 7C, color misregistration of the
yellow toner image against the black toner image in the image generated
in a trailing edge of the sheet is indicated in a positive region with
respect to a vertical axis. The misregistration of the yellow toner image
with respect to the black toner image is considered for the following
reason. According to the present exemplary embodiment, the yellow image
forming unit is a first station which performs the initial primary
transfer, and the black image forming unit is a fourth station which
performs the final primary transfer. The difference between the torques
on the drive roller when performing primary transfer is thus greatest
between the first station and the fourth station. In other words, the
load fluctuation is also the greatest between the first station and the
fourth station, so that color misregistration is significantly generated.

[0069]Referring to FIG. 7A, color registration is generated in the leading
edge of the first sheet. On the other hand, referring to FIG. 7C, color
misregistration is generated in the opposite direction as in the first
sheet in the trailing edge of the third sheet. The load torque on the
drive roller shaft decreases when the developing roller 54 comes into
contact with the photosensitive drum 26. The peripheral velocity of the
intermediate transfer belt 30 is thus greater when the black toner image
is primarily transferred as compared to when the yellow toner image is
primarily transferred, so that color misregistration is generated in the
leading edge of the first sheet as illustrated in FIG. 7A. Further, the
load torque on the drive roller shaft increases when the developing
roller 54 separates from the photosensitive drum 26. The peripheral
velocity of the intermediate transfer belt 30 is thus less when the black
toner image is primarily transferred as compared to when the yellow toner
image is primarily transferred. As a result, color misregistration is
generated in the trailing edge of the third sheet as illustrated in FIG.
7C.

[0070]Referring to FIG. 7B, color misregistration is hardly generated on
the second sheet when there is no fluctuation in the load torque in
performing the primary transfer. Further, color misregistration of the
magenta image and the cyan image with respect to the black image are
generated in the leading edge of the first sheet and the trailing end of
the third sheet (not illustrated). However, such misregistration is not
as significant as the misregistration between the yellow toner image and
the black toner image.

[0071]The above-described color misregistration is not generated when
there is no peripheral velocity difference between the photosensitive
drum 26 and the intermediate transfer belt 30. The present exemplary
embodiment thus describes a method for correcting the velocity of the
photosensitive drum 26 to reduce color misregistration.

[0072]As described above, the size of misregistration changes according to
the usage of the intermediate transfer belt 30 even when the peripheral
velocity difference between the photosensitive drum 26 and the
intermediate transfer belt 30 is the same (refer to FIG. 6). Therefore,
as illustrated in FIG. 8, a photosensitive drum velocity V(n) at which
color misregistration becomes zero cannot be acquired by only acquiring
one point, i.e., X1(V(1), R(1)), which indicates a relation between the
photosensitive drum velocity V(n) and the color misregistration R(n).
Therefore, according to the present exemplary embodiment, X2(V(2), R(2))
is acquired, and the photosensitive drum velocity V(n) at which color the
misregistration becomes zero is then acquired from the two points, i.e.,
X1(V(1), R(1)) and X2(V(2), R(2)).

[0074]Referring to FIG. 9, in step S1, the image forming control unit 12
drives the photosensitive drum 26 at a setting value V.

[0075]In step S2, the image forming unit 13 forms patches for detecting
the amount of color registration generated by the velocity fluctuation of
the intermediate transfer belt 30. In step S3, the registration detection
sensor unit 15 detects the patches. When forming the patches in step S2,
the image forming unit 13 forms the color registration pattern as
illustrated in FIG. 3B in response to an instruction from the image
forming control unit 12.

[0076]FIG. 10 illustrates a timing chart for performing the patch
formation (S2) and the patch detection (S3). Each of the operations
performed by the image forming apparatus is indicated on a vertical axis,
and time is indicated on a horizontal axis. The timing chart illustrated
in FIG. 10 will be described in detail below.

[0077]Referring to FIG. 10, at timing 130, timing 131, timing 132, and
timing 133, the image forming control unit 12 sequentially causes the
developing roller 54 of each color to come into contact with the photo
sensitive drum 26, starting with the yellow developing roller 54Y located
upstream. The image forming operation is thus started. After the black
developing roller 54Bk comes into contact with the photosensitive drum
26Bk at timing 133, the image forming control unit 12 outputs a Top
signal to perform patch formation at timing 134. The Top signal is output
after a predetermined time has elapsed from timing 133 and after the
velocity fluctuation of the intermediate transfer belt 30 becomes small.

[0078]At timing 135, the image forming unit 13 forms on the intermediate
transfer belt 30 yellow toner patches as illustrated in FIG. 3B. More
specifically, the image forming unit 13 forms LY1 in the left side on the
intermediate transfer belt 30 and RY1 in the right side on the
intermediate transfer belt 30. At timing 136, the image forming unit 13
forms black (i.e., a second color) toner patches LBk1 and LBk2, and RBk1
and RBk2 at equal intervals in front and in back of LY1 and RY1. Such
patches to be used in detecting misregistration are formed in a stable
state when the toner has entered the primary transfer nips of all colors,
and when there is no velocity fluctuation of the intermediate transfer
belt 30. Further, since the primary transfer positions are different for
the yellow toner image and the black toner image, the timing of forming
the black patches is delayed from the timing of forming the yellow
patches. An arrow B illustrated in FIG. 10 indicates such a delay in
time.

[0079]According to the present exemplary embodiment, the colors are
distinguished by referring to the toner color whose primary transfer
position is located most upstream as a first color, and the toner color
whose primary transfer position is located most downstream as a second
color. The first color is yellow and the second color is black according
to the present exemplary embodiment. However, the colors are not limited
to the above and depend on the arrangement of the photosensitive drums.

[0080]Further, as illustrated in FIG. 3B, three patches (marks), such as
LBk1, LY1, and LBk2, form one pattern. More specifically, a plurality of
patterns which are each a set of three patches is formed, and such
patterns are referred to as a first pattern, a second pattern, and a
third pattern in a case where it is necessary to distinguish the
patterns.

[0081]The formed black patches LBK1 and RBk1 then reach the detection
position of the registration detection sensor 90 (as indicated by an
arrow C illustrated in FIG. 10). At timing 137, the registration
detection sensor 90 detects a total of 6 rising and negative-going edges
of the formed patches. The registration detection sensor 90 detects the
midpoint of the detected rising edge and the down-going edge
corresponding to each patch as the position of the patch. The detection
process will be described in detail below with reference to FIG. 3C.

[0082]The intermediate transfer belt 30 is then rotated, and the
intermediate transfer member cleaner 32 cleans the previously formed
yellow and black patches LY1, RY1, LBk1, LBk2, RBk1, and RBk2. At timing
138, the image forming unit 13 forms yellow patches LY2 and RY2 (i.e.,
the first color mark) at a position which is an integral multiple of the
circumference of the photosensitive drum 26 from the position of the
yellow patches LY1 and RY1 and at a proximate same region (position)
after the intermediate transfer belt 30 is once rotated. An arrow A
illustrated in FIG. 10 indicates a length of approximately one rotation
of the intermediate transfer belt 30. A stable state is also reached at
timing 138, in which the toner has entered the primary transfer nip of
all colors, and there is no velocity fluctuation of the intermediate
transfer belt 30.

[0084]At timing 143, the image forming unit 13 then forms on the
intermediate transfer belt 30 black toner patches LBk3, and LBk4, and
RBk3 and Rbk4 at equal intervals in front and back of LY2 and RY2
respectively. The misregistration detection pattern (or the color
misregistration detection pattern) is thus formed by performing the toner
patch formation at timing 143 and timing 138. Further, the processes
performed at timing 138 and timing 143 are repeated if "NO" is determined
in step S8 and step S10 illustrated in FIG. 9 as will be described below.
Each of the patterns formed at timing 138 and timing 143 will be referred
to as a first pattern and a second pattern respectively.

[0085]At timing 143, the toner transiently enters a portion of the primary
transfer nips and does not enter the other primary transfer nips, so that
velocity fluctuation is generated in the intermediate transfer belt 30.
Further, at timing 143, a portion of the developing units (i.e.,
developing rollers) can be separated from or be in contact with the
photosensitive drum 26. Furthermore, the image forming unit 13 forms the
black toner patches LBk3, and LBk4, and RBk3 and Rbk4 similarly to the
yellow toner patches. More specifically, the image forming unit 13 forms
each of the black toner patches at a position which is located an
integral multiple of the circumference of the photosensitive drum 26 from
the position of the patches LBk1, LBk2, RBk1, and RBk2 and a proximate
same region (position) after the intermediate transfer belt 30 is once
rotated.

[0086]When the formed patches reach the detection position of the
registration detection sensor 90, the registration detection sensor 90
detects the position of each patch at timing 144.

[0087]According to the present exemplary embodiment, the patches LY1 and
the like are formed in the stable state and the patches LY2 and the like
are formed in the fluctuating state when the developing roller 54
separates from the photosensitive drum 26. These patches are positioned
apart by an integral multiple of the circumference of the photosensitive
drum 26 and are at a proximate same region (position) after the
intermediate transfer belt 30 is once rotated. This is to reduce the
effect of an edge-runout of the photosensitive drum 26 and the
non-uniformity in the film thickness of the intermediate transfer belt
30.

[0088]The edge-runout is generated due to difficulty of manufacturing the
photosensitive drum 26 having a uniform circumference. Further, it is
difficult to manufacture the intermediate transfer belt 30 of uniform
film thickness, so that the thickness becomes different, causing
difference in the conveyance velocity to be generated. To reduce the
effects of the edge-runout of the photosensitive drum circumference and
the unevenness in the film thickness of the intermediate transfer belt
30, the patches are thus formed at a distance which is an integral
multiple of the circumference of the photosensitive drum 26. Further, the
patches are formed at a proximate same region (position) after the
intermediate transfer belt 30 is once rotated. A cycle of the unevenness
in the film thickness is one circle of the intermediate transfer belt,
and it is not necessary to keep the position of the patch to be strictly
one cycle of the intermediate transfer belt 30.

[0089]As described above, the patches are formed at a position of an
integral multiple of the circumference of the photosensitive drum 26 to
reduce the effect of the edge-runout of the photosensitive drum 26.
However, the patches can also be formed at an integral multiple of the
circumference of the drive roller 100 to reduce the effect of the
edge-runout of the drive roller 100 which drives the intermediate
transfer belt 30. Further, the patches can be formed at a position which
is a common multiple of the circumferences of the photosensitive drum 26
and the drive roller 100.

[0090]Returning to the flowchart illustrated in FIG. 9, in step S4, the
image forming control unit 12 calculates the amount of color registration
from the difference in the timings of detecting the patches. The color
registration generated when there is no velocity fluctuation of the
intermediate transfer belt 30 is indicated as S. The color
misregistration generated when the developing roller 54 separates from
the photosensitive drum 26 is indicated as U.

[0091]The misregistration S is calculated by calculating color
misregistration in the left side on the intermediate transfer belt 30,
i.e., L1, and color misregistration in the right side on the intermediate
transfer belt 30, i.e., R1, using equations (7) and (8).

L1=LY1-(LBk1+LBk2)/2 (7)

R1=RY1-(RBk1+RBk2)/2 (8)

A mean value of the left-side color misregistration L1 and the right-side
color misregistration R1 is then calculated using equation (9) to
calculate the color misregistration S in a stable state where no velocity
fluctuation of the intermediate transfer belt 30 occurs.

S=(L1+R1)/2 (9)

The color misregistration S is caused by factors other than the tangential
force fluctuation generated at the primary transfer nip and corresponds
to the amount of static or direct color misregistration.

[0092]FIG. 3C illustrates the relative positions of the toner patches such
as LY1, LBk1, and LBk2. Referring to FIG. 3C, t1, t2, t3, t4, t5, and t6
indicate time required for the registration detection sensor 90 to detect
the edges of the patches from the reference position (reference timing),
and thus indicate the position of the patches. If LBk1=(t1+t2)/2,
LY1=(t3+t4)/2, and LBk2=(t5+t6)/2, then LY1-(LBk1+LBk2)/2 becomes zero or
proximately zero when there is no color misregistration. On the contrary,
if color misregistration is generated, LY1-(LBk1+LBk2)/2 does not become
zero. Further, the same can be said for other patches such as RBk1 and
RBk2, and therefore their description will be omitted.

[0093]The misregistration U generated when the developing roller 54
separates from the photosensitive drum 26 is calculated by calculating
color misregistration in the left side on the intermediate transfer belt
30, i.e., L2, and color misregistration in the right side on the
intermediate transfer belt 30, i.e., R2, using equations (10) and (11).

L2=LY2-(LBk3+LBk4)/2 (10)

R2=RY2-(RBk3+RBk4)/2 (11)

A mean value of the left-side color misregistration L2 and the right-side
color misregistration R2 is then calculated using equation (12) to
calculate the color misregistration U.

U=(L2+R2)/2 (12)

[0094]A difference P between the above-described color misregistration S
generated when the intermediate transfer belt 30 is stably moving and the
color misregistration U generated when the developing roller 54 separates
from the photosensitive drum 26 is then calculated using equation (13).
The calculated difference P which is color registration caused by the
velocity fluctuation of the intermediate transfer belt 30 is used to
correct the velocity of the photosensitive drum 26.

P=(S-U) (13)

According to the present exemplary embodiment, the color registration P is
detected three times in step S5 illustrated in FIG. 9, to improve the
detection accuracy of the color registration. In step S7, a mean value of
the color registrations P is calculated as color registration R to be
used in correcting the velocity of the photosensitive drum.

R=(P(1)+P(2)+P(3))/3 (13')

[0095]The method for correcting the velocity of the photosensitive drum
using the detected color misregistration average value R(n) will be
described below. In step S7, if the color registration average value
detected by the above-described method is R(1) and the peripheral
velocity of the photosensitive drum 26 is V (1), X1 (V(1), R(1))
illustrated in FIG. 8 can be acquired.

[0096]In step S8, if it is determined that the detected color
misregistration average value R(n) is less than a predetermined value
(YES in step S8), the process proceeds to step S9. In step S9, it is
determined that the peripheral velocity difference between the
photosensitive drum 26 and the intermediate transfer belt 30 is small.
The velocity of the photosensitive drum 26 is thus not corrected, and the
current velocity of the photosensitive drum 26 is employed. However, the
velocity of the photosensitive drum 26 can be corrected even if the color
misregistration average value R(n) is small to reduce the peripheral
velocity difference.

[0097]If the detected color misregistration average value R(n) is greater
than a predetermined value (NO in step S8), the process proceeds to step
S11. In step S11, the velocity of the photosensitive drum 26 is changed
to detect a color misregistration average value R(2) using a
photosensitive drum velocity V(2) which is different from the
photosensitive drum velocity V(1). If the color misregistration average
value R(1) is greater than zero, the peripheral velocity of the
photosensitive drum 26 is decreased by 0.1%. On the other hand, if the
color misregistration average value R(1) is less than zero, the
peripheral velocity of the photosensitive drum 26 is increased by 0.1%.
According to the present exemplary embodiment, the photosensitive drum
velocity V(2) is different from the photosensitive drum velocity V(1) by
0.1%. It is preferable to set the photosensitive drum velocity V(2)
within a range in which there is a linear relation between the velocity
of the photosensitive drum 26 and the color misregistration.

[0098]The color misregistration average value R(2) when the peripheral
velocity of the photosensitive drum 26 is V(2) is then calculated
similarly to the color misregistration average value R(1) in step S2 to
step S7.

[0099]In step S13, a drum velocity correction coefficient C is calculated
using equation 14 and the acquired X1 (V(1), R(1)) and X2 (V(2), R(2)).
The drum velocity correction coefficient C is a parameter which indicates
a velocity correction amount per unit misregistration amount. In other
words, the drum velocity correction coefficient C is an amount of change
in the X-axis direction when there is a unit amount of change in the
Y-axis direction.

C=(V(1)-V(2))/(R(1)-R(2)) (14)

[0100]In step S14, the photosensitive drum velocity V when there is no
color registration, i.e., when the peripheral velocity difference between
the photosensitive drum 26 and the intermediate transfer belt 30 is zero
or proximately zero, is calculated using the calculated drum velocity
correction coefficient C. Equation 15 is used to calculate the
photosensitive drum velocity V. The velocities of one or more motors
driving the photosensitive drum are thus comprehensively corrected using
the velocity calculated by equation (15), and hereinafter, the image
forming process is performed at the corrected photosensitive drum
velocity.

V=V(1)-C×R(1) (15)

[0101]As described above, the peripheral velocity V of the photosensitive
drum 26 is corrected. However, the method for correcting the peripheral
velocity is not limited to the above-described method. Any method can be
used as long as the relative velocity between the image bearing member
(i.e., the photosensitive drum) and the intermediate transfer member
(i.e., the intermediate transfer belt) is corrected to zero or
proximately zero. The traveling velocity of the intermediate transfer
belt can also be corrected by reflecting the difference between the
velocity V acquired using equation (15) and the velocity V before
correction.

[0102]In other words, color misregistration can be flexibly reduced when
forming the image without shortening the life of the image forming unit
by correcting the velocity of either the image bearing member (i.e., the
photosensitive drum) or the intermediate transfer member (i.e., the
intermediate transfer belt). The present invention can thus provide a
method which takes into consideration the effect of the degree of change
in the tangential force between the image bearing member (i.e., the
photosensitive drum) and the intermediate transfer member (i.e., the
intermediate transfer belt).

[0103]Further, in the above-described exemplary embodiment, the drum
velocity correction coefficient C is calculated based on two points,
i.e., X1 (V(1), R(1)) and X2 (V(2), R(2)). However, the drum velocity
correction coefficient C can be calculated based on more than two points.
An effect of scattering in Xn (V(n), R(n)) can be reduced by calculating
the drum velocity correction coefficient C based on a plurality of
points, and the accuracy of the drum velocity correction coefficient C
can be improved. The photosensitive drum velocity correction sequence
determines the correction amount based on the drum velocity correction
coefficient C. The accuracy in correcting the velocity of the
photosensitive drum 26 can thus be improved by improving the accuracy of
the drum velocity correction coefficient C, so that the color
misregistration can be reduced.

[0104]Furthermore, as illustrated in FIG. 11, the drum velocity correction
coefficient C is expressed as a gradient of the linear line by taking the
color misregistration on the vertical axis and the velocity of the
photosensitive drum 26 on the horizontal axis. As described above, the
tangential force acting on the primary transfer nip changes according to
the usage of the intermediate transfer belt 30 even when the peripheral
velocity difference is the same. Color misregistration thus increases as
the usage of the intermediate transfer belt 30 increases. As a result,
the drum velocity correction coefficient C changes according to the usage
as illustrated in FIG. 11. In the initial state, the gradient is small,
and the gradient increases as the usage increases.

[0105]According to the present exemplary embodiment, the drum velocity
correction coefficient is calculated when correcting the velocity of the
photosensitive drum 26. The velocity of the photosensitive drum 26 can
thus be corrected to reduce the color misregistration regardless of the
usage of the intermediate transfer belt 30. Moreover, the velocity of the
photosensitive drum 26 can be corrected even when the drum velocity
correction coefficient changes due to factors other than the usage of the
intermediate transfer belt 30, such as the usage environment of the
apparatus.

[0106]As described above, according to the present exemplary embodiment,
color registration can be reduced without immoderately shortening the
life of the image forming unit and by flexibly suppressing velocity
fluctuation of the intermediate transfer member generated while forming
an image. In other words, the present invention can provide a method
which takes into consideration the effect of the degree of change in the
tangential force between the image bearing member (i.e., the
photosensitive drum) and the intermediate transfer member (i.e., the
intermediate transfer belt).

[0107]If the circumference of the drive roller 100 which determines the
conveyance velocity of the intermediate transfer belt 30 is a designed
central value, the peripheral velocity difference of the photosensitive
drum 26 and the intermediate transfer belt 30 can be previously set to be
zero or proximately zero. However, since there is dispersion in the
circumference of the drive roller 100 within the range of tolerance, the
velocity of the intermediate transfer belt 30 changes by an amount of the
difference from the designed central value. Therefore, the peripheral
velocity difference is generated between the photosensitive drum 26 and
the intermediate transfer belt 30 and causes color misregistration.

[0108]To address this problem, the sequence illustrated in FIG. 10 is
performed when the image forming apparatus is initially activated. As a
result, the velocities of the photosensitive drum and the intermediate
transfer belt can be matched even when the circumferences of the
photosensitive drum and the driving roller are different from the
designed central values. The generation of the color misregistration can
thus be reduced. Further, color misregistration can be reduced by
performing the sequence illustrated in FIG. 10 when changing the process
cartridge or the intermediate transfer belt unit 31.

[0109]A second exemplary embodiment of the present invention will be
described below. According to the first exemplary embodiment, patch
formation at timing 135 and timing 136, and patch detection at timing 137
are repeatedly performed. However, the present invention is not limited
to such a method.

[0110]According to the present exemplary embodiment, patch formation at
timing 135 and timing 136, and patch detection at timing 137 can be
omitted when performing step S2 to step S7 either for the first time or
for the second time. The value of S=(L1+R1)/2 acquired by performing step
S2 to step S7 either for the first time or the second time can be used
instead. More specifically, the pattern including the patches formed at
timing 135 and timing 136 illustrated in FIG. 10 may be formed to
correspond to at least one of the patterns formed by performing the
processes at timing 138 and timing 143 for the first time and for the
second time.

[0111]The image forming control unit 12 can acquire the color
misregistration amount for each relative velocity similarly to the first
exemplary embodiment by using the detection result of the patches formed
at timing 135 and timing 136 as described above.

[0112]The image forming control unit 12 can calculate the direct color
misregistration amount which is not caused by the tangential force
fluctuation generated at the first transfer nip, by using the detection
result of the patterns formed at timing 135 and timing 136.

[0113]Further, the image forming control unit 12 subtracts (deletes) the
calculated direct color misregistration amount from the detection result
of the pattern formed in performing the processes at timing 138 and
timing 143 for the first time and the second time illustrated in FIG. 10.
As a result, the color misregistration amount caused by the tangential
force fluctuation generated at the first transfer nip for each relative
velocity can be extracted. After extracting the color misregistration
amount, the peripheral velocity difference (relative velocity) between
the photosensitive drum 26 and the intermediate transfer belt 30 is
corrected based on the extracted result, similarly to the first exemplary
embodiment. The detailed processes to follow are similar to the first
exemplary embodiment, and description will be omitted.

[0114]A third exemplary embodiment according to the present invention will
be described below. According to the first exemplary embodiment, the
color misregistration S is calculated when there is no change in the
tangential force, i.e., when the intermediate transfer belt 30 is
rotating in a stable state. However, the position for forming the pattern
may be corrected before performing the color misregistration detection
sequence illustrated in FIG. 9 so that the color misregistration becomes
zero. The calculation of the color misregistration S can then be omitted.

[0115]In such a case, the process illustrated by the flowchart of FIG. 9
can be performed by omitting the processes performed at timing 135,
timing 136, and timing 137 illustrated in FIG. 10, and the calculations
using equations (9) and (13). The time necessary to perform the toner
patch formation and the detection can then be reduced by previously
correcting the color misregistration and executing the modified process
of the flowchart illustrated in FIG. 9. The color misregistration
correction to be previously performed uses a known technique. More
specifically, the toner patches to be used in correcting the color
misregistration for four colors are formed, and the position of an
adjusting color (e.g., colors other than yellow) with respect to a
reference color (e.g., yellow) is corrected. A detailed description will
thus be omitted.

[0116]Further, the processes performed at timing 135, timing 136, and
timing 137 can also be omitted if the process illustrated in the
flowchart illustrated in FIG. 9 according to the first exemplary
embodiment is performed when there is no color misregistration (i.e., S
calculated using the above-described equation (9) is zero).

[0117]As described above, the present exemplary embodiment at least forms
both the yellow toner patch in the stable state in which the toner enters
all primary transfer nips (at timing 138) and the black toner patch in
the fluctuating state in which the toner enter a portion of the primary
transfer nips (at timing 143). The patch formation performed at timing
135 and timing 136 illustrated in FIG. 10 simplifies the color
misregistration correction which previously sets the color
misregistration S to zero and also improves user-friendliness.

[0118]A fourth exemplary embodiment will be described below. The first,
second, and third exemplary embodiments are directed to a method for
detecting color misregistration generated when the developing roller 54
separates from the photosensitive drum 26. The color misregistration P
can also be calculated by detecting the color misregistration generated
when the developing roller 54 comes into contact with the photosensitive
drum 26.

[0119]More specifically, at timing 130 illustrated in FIG. 10, only the
yellow developing roller 54Y in the developing unit 51Y comes into
contact with the photosensitive drum 26Y. At timing 135, yellow patch
formation is then performed in the fluctuating state when the velocity
fluctuation is generated in the intermediate transfer belt. Further, at
timing 136, black patch formation is performed in the stable state in
which all developing units 51 are in contact with the respective
photosensitive drums 26.

[0120]Furthermore, the patch formation at timing 138 and timing 143 are
performed in the stable state in which all developing units 51 are in
contact with the respective photosensitive drums 26. The process
illustrated in the flowchart of FIG. 9 is then performed according to the
above-described changes in the processes illustrated in FIG. 10. In such
a case, the patch formation performed at timing 138 and timing 143 can be
omitted, or one of the two series of processes performed at timing 138,
timing 143, and timing 144 can be omitted similarly to the first, second
and third exemplary embodiments.

[0121]As described above, color misregistration can be reduced without
immoderately shortening the life of the image forming unit when the
developing unit 51 starts to come into contact with the photosensitive
drum in addition to when separating from the photosensitive drum. For
example, the color misregistration can also be reduced when primary
transfer of a first page of a print job is started. In other words, the
present invention can provide a method which takes into consideration the
effect of the degree of change in the tangential force between the image
bearing member (i.e., the photosensitive drum) and the intermediate
transfer member (i.e., the intermediate transfer belt).

[0122]A modified example according to the present invention will be
described below. In the above-described photosensitive drum velocity
correction sequence, the velocity of the photosensitive drum 26 is
corrected so that the color misregistration becomes zero, i.e., the
peripheral velocity difference between the photosensitive drum 26 and the
intermediate transfer belt 30 becomes zero or proximately zero. However,
since the peripheral velocity difference between the photosensitive drum
26 and the intermediate transfer belt 30 affects also transfer
efficiency, a predetermined peripheral velocity difference may become
necessary between the photosensitive drum 26 and the intermediate
transfer belt 30. More specifically, the toner on the photosensitive drum
26 can be more easily scraped off when there is a predetermined
peripheral velocity difference, and the transfer efficiency is thus
improved.

[0123]Since the relation between the peripheral velocity difference and
the color misregistration can be acquired by calculating the drum
velocity correction coefficient C, an arbitrary peripheral velocity
difference can be set. Therefore, a relation between the velocities of
the photosensitive drum 26 and the intermediate transfer belt 30 which
takes into account the color misregistration and the transfer efficiency
can be set by performing the photosensitive drum velocity correction
sequence.

[0124]Further, the velocity of the intermediate transfer belt 30 can be
corrected using a method similar to correcting the velocity of the
photosensitive drum 26.

[0125]Furthermore, the velocities of the photosensitive drum 26 and the
intermediate transfer belt 30 may become different from the designed
central values by a change in the environmental temperature, or the
temperature inside the apparatus when papers are continuously passed
through the apparatus. In such a case, a temperature detection unit is
disposed inside the apparatus main body or near the photosensitive drum
or the driving roller. When a predetermined temperature rise is detected,
the photosensitive drum velocity correction sequence is performed to
prevent color misregistration. Similarly, the velocity fluctuation due to
the usage of the intermediate transfer belt 30 can be corrected based on
a pixel count or a history of the number of passing sheets.

[0126]Moreover, the velocities of the photosensitive belt and an
intermediate transfer drum employed as the image bearing member in an
image forming apparatus can be corrected by a similar velocity correction
sequence.

[0127]While the present invention has been described with reference to
exemplary embodiments, it is to be understood that the invention is not
limited to the disclosed exemplary embodiments. The scope of the
following claims is to be accorded the broadest interpretation so as to
encompass all modifications, equivalent structures, and functions.